1. Field of the Invention
The present invention relates to contact lenses, and more particularly to a bifocal lens having a soft plastic material in contact with the eye.
2. Description of the Prior Art
Persons suffering from presbyopia require bifocal type lenses to enable them to focus on both distant and near objects. Such lenses require areas having two different powers and correction. Bifocal lenses are known for both conventional glasses, and for contact lenses that are inserted directly onto the eye. A common bifocal contact lens is of the alternating vision type in which the distance power portion is ground to permit the target to be viewed when looking straight ahead. For closeup viewing, the lens must move over the surface of the eye to place the closeup power portion directly over the pupil, with the distance power portion then out of the line of sight.
It is, therefore, common to form the lens from a rigid plastic material, such that the lower edge of the lens contacts the lower eyelid when the eye is in down gaze, causing the lens to move upward. Gravity and the upper eyelid return the lens to the distance position when the eye is in primary gaze. It has been found that many persons have difficulty with a lack of comfort of hard plastic lenses, and prefer those of soft materials. Soft plastic translating bifocal lenses of the prior art have met with limited success. In the instance where the soft plastic bifocal lens of the prior art did not have a lower portion that extended under the lower eyelid when positioned on the patient, translation of the lens beneath the lower eyelid resulted in irritation and consequent discomfort to the patient. In the instance where the soft plastic bifocal lens of the prior art was disposed beneath the lower eyelid, rotation of the eyeball to a down gaze for closeup viewing did not produce sufficient stability of the lens and the lower eyelid so as to properly position the pupil relative to the near correction portion of the lens.
Typical prior art patents of bifocal contact lenses include U.S. Pat. Nos. 4,874,234; 4,618,229; 3,597,055; 3,560,598; and 3,440,306, and Japanese Patent No. 63-95,415.
U.S. Pat. No. 5,071,244 describes a soft hydrophilic contact lens having its central portion formed to the distance correction prescription of the user. A small auxiliary lens is formed to the near vision prescription of the user, said auxiliary lens being cemented along a lower margin of the main lens. Further, the auxiliary lens protrudes slightly from the surface of the main lens, the main lens being formed to fit over the eye of the user with the central portion centered over the pupil. Thus, the near vision lens will normally be below the line of sight of the wearer.
When the wearer looks downward for reading or the like, the tapered margin of the carrier may tend to move beneath the lower eyelid. When the lower eyelid contacts the protruding auxiliary lens, the carrier is translated superiorly or vertically, placing the auxiliary lens over the pupil, and the line of sight of the user then passes through the near vision lens portion.
When the user returns the eye to a primary gaze, the lens will translate downward, or inferiorly, by gravity, and the action of the upper lid, to move the distance correction portion of the lens back over the pupil.
In the lens of U.S. Pat. No. 5,071,244 ('244), the auxiliary lens 12, which is used to translate the lens over the eyeball during the conversion from distance correction to near correction, has a limitation to its thickness which is a reflection of the diopter value needed to correct the near vision problem. Further in the '244 patent, the placement of the translation bead 12 of the lens for near vision correction dictates a significant degree of translation of the contact lens in order to align the pupil and the near vision correction portion of the overall lens. The lens 10 of the '244 patent is of uniform thickness at its edges 14. As a consequence, the balanced thickness of the top and bottom edges of the prior art lens, works against the lens returning from its position on the eyeball at the near vision correction mode to its position on the eyeball during the distance vision correction mode.
A contact lens having a fixed position on the eyeball is described in U.S. Pat. No. 4,614,413.
The lens of the subject invention: (1) minimizes the translation distance in going from the distance vision correction mode to the near vision correction mode; (2) is not dependent on the near vision correction parameter of the user in determining the size of the protrusion from the lens surface that is the translating portion of the lens and (3) has greater stability, i.e., will more easily return to the distance vision correction position on the eyeball than the prior art bifocal contact lens.